The present invention relates generally to patient defibrillation apparatus and means for producing and displaying selected parameters during defibrillation.
A defibrillator is commonly used to administer a high voltage, direct current shock pulse through a pair of electrodes, or "paddles", which are placed in contact with the chest of a patient in cardiac distress. A selected, discrete quantity of energy is stored in a capacitor and then electrically discharged into the patient through the paddle circuit. The quantity of energy is typically selected on the basis of patient size, weight and condition.
One type of known defibrillator monitors and displays the magnitude of peak current in the discharge pulse during defibrillation. Measured peak current is one parameter used in assessing the performance of the defibrillator. Another useful parameter is the quantity of energy actually delivered to a patient during defibrillation. Known defibrillators display a parameter often termed "delivered energy"; however, the displayed quantity is in reality only the anticipated energy that would be applied to a patient in an assumed case. For example, it is assumed that the patient represents some selected nominal resistive load, say 50 ohms. Since the selected nominal load rarely matches actual load resistance, the energy value displayed is not an accurate representation of the energy actually delivered to a patient during defibrillation.
Actual external load resistance produced at the paddles during defibrillation is referred to as the patient's transthoracic resistance. Researchers have attempted to estimate this parameter from calculations of the integral of the output voltage waveform over time at the defibrillator paddles during discharge of the defibrillator storage capacitor. Such work has been reported by Ewy, et al., in a paper entitled "Canine Transthoracic Resistance", published in the Journal of Applied Physiology, Vol. 32, No. 1, January, 1972. The process of obtaining a voltage-time integral is difficult because it requires high speed, complex measuring and integration circuits.
Known defibrillators are incapable of monitoring and displaying the value of patient load resistance which occurs when the defibrillation shock pulse is administered. To the defibrillator operator, patient load resistance is a useful parameter for assessing the efficiency of defibrillation.